1. Field of the Invention
The present invention relates to an element mounting structure and an element mounting method in which an element to be used for optical communications is mounted on a carrier made with a material such as ceramic, with stability and high-reliability.
2. Description of the Related Art
An element to be used for the optical communications and the like is mounted on a carrier such as a ceramic carrier. In order to mount the element on the carrier, solder mounting is normally adopted, considering ensuring reliability. In the solder mounting, it is important to remove an oxide film.
As an element for the optical communications, a bare chip of light receiving elements (PIN-PD, APD) is used. The light receiving element bare chip has an electrode with small size, such as 20˜100 μm in diameter φ.
When the element for the optical communications is mounted on the carrier with solder mounting, fluxing, which is a general method for removing the oxide film when an electrical component is mounted using solder, cannot be used because the element might be contaminated. Moreover, the electrode of the light receiving element for the optical communications is too small to adopt a normal scrub.
So, as shown in Japanese Patent Application Laid-open No. 2001-021616 (Patent Document 1), a stage is finely moved by a micromotion unit, so that the scrub occurs when the bare chip is mounted, and then the element is mounted on the carrier. Alternatively, the scrub occurs by utilizing ultrasound, and then the element is mounted on the carrier.
Adopting the method in Patent Document 1, problems are caused when the element is mounted on the carrier. The problems will be explained with reference to drawings. As shown in FIG. 3, a light receiving element bare chip 21 is provided with a p-type electrode 22 and n-type electrodes 23. A carrier 24 is provided with a p-type electrode pad 25 and n-type electrode pads 26.
On the occasion of mounting, the p-type electrode 22 of the light receiving element 21 is positioned at the p-type electrode pad 25 on the carrier 24, and the n-type electrodes 23 are positioned at the n-type electrode pads 26 respectively, however, they cannot be scrubbed either in a X or a Y direction because the p-type electrode 22 and the p-type electrode pad 25, as well as the n-type electrodes 23 and the n-type electrode pads 26, are in the same size in each pair. Accordingly, the scrub is performed with the micromotion or the ultrasound upon mounting.
Therefore, the oxide film cannot be removed completely with the mounting method shown in Patent Document 1, and there remains a problem where stable mounting cannot be realized. Further, when the ultrasound is adopted to cause the scrub instead of the micromotion according to Patent Document 1, there is a problem in which the ultrasound damages the light receiving element.